Add Guzzle 3 support

As mentioned in [this issue] [issue], this library isn't ideally suited for signing requests that will be made to a third party. 

Given the prevalence of Guzzle 3.x within the 99designs stack, this PR:

- Adds a wrapper for Guzzle Request objects that allow them to be signed/verified as if they were Symfony Request objects.
- Adds a plugin for Guzzle that automatically signs requests using a given context object
- Relevant tests for added functionality.

There should be no BC breaks.

**Issues:**

Maybe this should be a separate library that can be pulled into projects that require the ability to make requests across stacks?

[issue]: https://github.com/99designs/http-signatures-php/issues/2